The number of different radios in mobile communication devices is steadily increasing to facilitate more flexible connectivity and a broader range of services. Cellular access alone is no longer sufficient, but new wireless technologies are integrated into communication devices today and especially in the future to enable novel connectivity solutions. Integration of multiple radios into a single terminal, however, introduces a serious integration challenge that is becoming more pronounced as the number of radios increases. One element of the integration challenge is the appropriate handling of simultaneous operation of radios. It is quite evident that users are willing to use different radios at the same time, like using a headset employing wireless Bluetooth® technology during a GSM phone call, using a wireless local area network (WLAN) connection for Internet surfing while using a frequency modulation (FM) transmitter for listening to stored sound and music files at a remote FM receiver, for example.
If two or more operational radio connections are provided from one communication device, the connections may very well interfere with one another. Even if the connections are not operating on the same frequency band, they may still interfere with each other due to non-ideal properties of components of the communication device. The components may introduce spectral leakage, and the selectivity of receivers may not be ideal, meaning that they may also receive signal components belonging to a signal other than the desired one. Especially harmonics of a transmission frequency and mixing products are likely to cause interference as they often peak in spectral power outside the desired band. An analysis of harmonics can be done by calculating integer multiples of a transmission frequency or frequency used in the transmitter, for example an oscillator frequency. The analysis of mixing products is more complex, as two or more frequencies are involved. Typically, the sums and differences of the two or more frequencies (f1, f2, etc.) are calculated, including sums and differences of harmonics, e.g. f1+2*f2. All transmissions should be taken into account for the analysis, including unwanted transmissions by components in a device that are not meant to radiate, like memory card readers or buzzers.
If a number of connections operate simultaneously in the same band, the interference they cause to one another is much more severe than if they were operating in separate bands. These connections cause inter-system interference to one another, which may result in a degraded quality of service. This may happen because both connections operate from the same communication device, and thus the radio transceivers may be located within a few centimeters from each other. They may also be using the same radio components, like an antenna, for instance.
If only non-continuous transmitters, i.e. transmitters that send out data in time slots, are involved, a scheduler which schedules radio communication processes may avoid such inter-system interference by putting the transmission(s) and/or reception(s) in adjacent time-slots, thus avoiding simultaneous operation. However, if a continuous, or a substantially continuous, transmission is involved, a different solution has to be found.
Here, a substantially continuous transmission is understood to be a transmission in which the transmitter is active all the time or in which the activity of the transmitter occupies so much time that no time slots are left over for other wireless communication interfaces to operate. Thus, a continuous transmission is a transmission that is incompatible with slotted transmissions of other radio units, so that simultaneous transmission cannot be avoided.
For example, a substantially continuous transmitter may be required to switch off the transmitter for 10 microseconds in regular or irregular intervals. As transmission bursts of TDMA or TDD systems are longer than 10 microseconds, the scheduler cannot put transmissions of the substantially continuous transmitter and the TDMA or TDD system in adjacent time-slots in order to avoid simultaneous operation.